Disc drives enable users of computer systems to store and retrieve vast amounts of data in a fast and efficient manner. The data are stored on one or more circular discs having a selectively magnetizable media accessed by a corresponding array of read/write heads of a controllably positionable actuator.
A leading failure mode of disc drives is mechanical failure of the magnetic media due to contact between the discs and actuator and/or heads when the disc drives are subjected to external mechanical shocks. It is generally recognized that resistance to such failures is an inherent material property of the media, favorably correlated to media hardness as indicated by impact response. Thus, by specifying an acceptable dynamic hardness threshold for a magnetic disc, such failures can be substantially minimized. Measurement and characterization of the impact response of a magnetic disc, to the extent that it is indicative of dynamic hardness, is thus a worthy objective in support of a disc drive manufacturer's quality and reliability goals.
Impact methods using spherical balls have been used for a number of years to determine the hardness of various materials. Extensive efforts have been extended in developing analytical interpolation approaches which compare the characteristic response of tested materials to the response of a known material. These efforts have lead to the successful development of relatively aggressive test methods using metallic balls several millimeters in diameter which impact the target material at velocities of several meters per second, typically leaving craters several micrometers deep. Such conditions as these are relatively easy to control and the characteristic responses are relatively easy to measure and correlate.
The present state of the art, however, is ill-suited to the testing of magnetic disc media. The impact conditions of the existing art are prohibitively aggressive for purposes of evaluating the physical properties of the top surface layers of a disc drive disc, as these layers are, collectively, typically less than a micrometer thick. Because the magnitude of a detent in the target material created by an impacting ball is directly proportional to the amount of energy in the falling ball, less aggressive impact conditions necessitate the use of lighter and smaller balls and lower drop heights in order to evaluate such media substrates. However, the lack of precision in the prior art introduces significant error in the repeatability of detent characteristics, where the characteristics of concern are measured in terms of angstroms. That is, at the very low drop heights required to make indentations as shallow as described here, small errors in height and non smooth releases can cause large variations in crater depths. Therefore, accurate height positioning and smooth release are critical. Prior art impact testers introduce excessively deep craters and excessive amounts of variability with regard to ball release dynamics, ball positioning, disc support and the like, making characterization of physical properties of magnetic media impractical. More importantly, it has not been possible to make sub-micrometer deep indents using the larger balls even at very low drop heights because of too much energy in balls greater in size than one millimeter even at one to two millimeter drop heights.
Accordingly, there is a need for advancements in the art whereby the physical properties of highly sensitive materials, such as the magnetic media of disc drive discs, can be precisely and reliably determined.